The present invention relates to the field of electroreprography. In particular the invention relates to compositions with utility in electroreprography. The present invention, therefore, relates to novel toner compositions, containing certain charge control agents, which have superior properties which render them particularly useful in electroreprography.
Electroreprography is any process in which an image is reproduced by means of electricity and incident radiation, usually electromagnetic radiation more usually visible light. Electroreprography includes the technology of electrophotography which encompasses photocopying and laser printing technologies. In both these technologies a latent electrostatic image in charge is produced by exposure of a photoconductive drum to light. This can be either reflected light from an illuminated image (photocopying) or by scanning the drum with a laser usually under instruction from a computer (laser printing). Once a latent image has been produced in charge it must be developed with colorant so that a visible image can be printed onto paper.
Toner compositions are used to develop the latent image on the drum into a visual image. During use in an electroreprographic device friction between particles of toner, their carrier and/or parts of the device in which the toner is used causes the toner particles to become charged with an electrostatic charge (tribocharge). The exact mechanism of development of the toner image will then vary according to the specific device used. For example, in a conventional photocopier the toner composition may be formulated so that tribocharged toner particles will be opposite in sign to the latent image on the drum and toner will be attracted to the latent image on the drum to develop an image in toner on the drum which corresponds to the original document. The developed image is then transferred to a substrate such as paper (e.g. by a pressure roller and/or voltage). The transferred image is fixed to the substrate (e.g. by heat) to produce a hard copy of the image. The image drum is then cleaned and the device is ready to produce the next copy. Thus toner compositions are used both to develop the latent image on the drum and to produce the final hard copy.
Thus it is desirable for toner compositions to comprise particles which can possess readily an electrostatic charge (tribocharge) so they can be attracted to the latent image on the drum to develop the latent image. Toners which readily tribocharge may also have the further advantage of facilitating rapid and more complete removal of any residual toner from the image drum (e.g by electrostatic repulsion). This may improve image quality (by reducing ghost images from previous copies) and may reduce the cycle time between copies and thus increase the speed of copying.
It has been found that the addition of certain charge control agents (hereinafter known as CCA's) to toner compositions helps the production and stability of triboelectric charge within the toner. Use of CCA's may also lead to improved image quality when the latent image is transferred to the paper. The mechanism for the action of CCA's is unclear, but the industry continues to seek compounds with improved abilities as CCA's.
Properties desired in ideal CCA's; toner compositions to which they are added; and/or the hard copies they produce are well known to those skilled in the art. Such properties might comprise any or all of the following: ability to stabilize larger tribocharge; improved tribocharge distribution and/or uniformity of charge within an individual toner particle and/or across the population of toner particles within a toner composition; reduced cost, reduced toxicity or non-toxicity, greater stability under conditions of use, good compatibility with the binder resin in a toner, improved image resolution, greater speed of image production, reduction in print bleed in the hard copy and/or improved colorant properties.
The CCA's currently available are not completely satisfactory in some or all of these respects. Thus it would be desirable to provide CCA's which result in improvements in some or all of the preceding areas.
Naphth[enoxyphenoxy] azo complexes with chromium are known as CCA's for negative toners for example from JP (Kokoku) 61-045,229 and JP (Kokoku) 62-034,516 (both Nippon Kayaku KK).
It had been thought that to be particularly effective as CCA's such metal complexes should comprise chromium, as complexes with other metals were thought to be less effective at stabilizing tribocharge. This was especially true in toners which become negatively charged during use and hence require a CCA which is effective at stabilizing negative electrostatic charge (hereinafter known as "negative CCA's"). However use of heavy metals such as chromium has well known disadvantages.
EP 0,664,493-A (Hodogaya) describes use of a variety of Fe, Co and Cr metal azo complexes which are useful as CCA's for stabilizing positively charged toners. This teaches away from their use as CCA's in negative toners.
Surprisingly and contrary to what would be expected, the applicant has discovered that simple salts of certain iron and cobalt metal complexes are more effective as negative CCA's for stabilizing negatively charged toners than prior art CCA's which are chromium complexes. Toner compositions comprising such CCA's overcome some or all of the above stated disadvantages of the prior art and in particular are substantially free of chromium.